A Mannesmann piercing method has been widely known as a method for producing a seamless pipe. In this method, first, a material to be pierced (round billet) that is heated to a certain temperature is subjected to a piercing process with a piercing mill to obtain a hollow shell. Subsequently, the wall thickness is decreased by using an elongating mill such as an elongator, a plug mill, or a mandrel mill. Furthermore, reheating is performed when necessary and then the outer diameter is mainly decreased with a stretch reducing mill or a sizing mill to obtain a seamless pipe having a predetermined size.
Examples of a known piercing mill include a Mannesmann piercer in which a pair of inclined rolls, a piercing plug, and two guide shoes are combined; a three rolls piercer in which three inclined rolls and a piercing plug are combined; and a press roll piercer in which two grooved rolls and a piercing plug are combined. In the piercing process that uses such a piercing mill, a tool (plug) for a piercing mill is exposed to a high-temperature and high-load environment for a long time and wear, erosion, and the like are easily caused. Therefore, as described in Patent Literatures 1, 2, 3, 4, and 5, the wear of a tool for a piercing mill has been prevented by forming an oxide scale having a thickness of several tens of micrometers to several hundred micrometers on a surface of the tool through an oxide scale-forming heat treatment at high temperature.
In recent years, however, there has been an increasing demand for high-alloy steel seamless pipes made of, for example, 13Cr steel and stainless steel that have high hot deformation resistance and a surface on which an oxide scale is not easily formed. The technologies described in Patent Literatures 1, 2, 3, 4, and 5 pose a problem in that, when such a high-alloy steel is pierced, a tool is quickly worn.
In view of the foregoing problem, the inventors of the present invention have proposed a tool for a piercing mill with excellent wear resistance in Patent Literature 6. In the technology described in Patent Literature 6, the tool has a composition containing C: 0.05% to 0.5%, Si: 0.1% to 1.5%, Mn: 0.1% to 0.5%, Cr: 0.1% to 1.0%, Mo: 0.5% to 3.0%, W: 0.5% to 3.0%, and Nb: 0.1% to 1.5% and further containing Co: 0.1% to 3.0% and Ni: 0.5% to 2.5% such that (Ni+Co) satisfies less than 4% and more than 1%. The tool has a scale layer in the surface layer thereof and the scale layer includes a net structure scale layer complicatedly intertwined with a metal on the substrate steel side. Furthermore, the tool for a piercing mill includes a microstructure containing a ferrite phase at an area fraction of 50% or more, the microstructure being formed on the substrate steel side from the interface of the scale layer. This can increase the lifetime of the tool and improves the productivity of high-alloy steel seamless pipes with a piercing mill.